Topographic and geologic evolution of fracture zones

Abstract

The age difference of oceanic crust on opposite sides of a fracture zone results in a depth differential and in different subsidence rates on the two sides. There is thus a component of dip-slip motion along the entire length of a fracture zone that decreases away from the central transform segment. The relief across a fracture zone will also change in a simple way due to the √age-dependence of the depth of oceanic crust. This dependence can also be expressed in terms of a (depth) 2 v. distance relationship that may be useful in identifying ambiguous ocean floor features as ancient fracture zones. Application of this procedure to the bathymetry of the well-studied Oceanographer Fracture Zone predicts spreading half-rates (12–13 mm/yr) in good agreement with the rate inferred from magnetic anomaly spacing; the model is not consistently successful, however, in predicting ridge axis positions or length of the transform offset. The relief across a fracture zone is also a factor in the evolution of sedimentary fill in the elongate bathymetric trough typical of many fracture zones. On the basis of the general geometric, structural, and igneous evolution of a fracture zone, events in the ridge–ridge transform segment, we have predicted the stratigraphic and structural relations that might be preserved at positions progressively distant from the spreading ridges. Other major complexities may arise from events such as ridge jumps or changes in rotation poles. The close spacing of fracture zones along some spreading ridges suggests that they could be preserved in ophiolite complexes, and stratigraphic and structural relations similar to those predicted have been observed in ophiolites from Newfoundland, Turkey, Cyprus, and the Apennines.